The present disclosure relates to ultrasonic imaging. More particularly, the invention relates to a system and method that permits uniform harmonic imaging and uniform contrast agent detection and destruction.
Tissue Harmonic Background
Ultrasonic imaging has quickly replaced conventional X-rays in many clinical applications because of its image quality, safety, and low cost. Ultrasonic images are typically formed through the use of phased or linear array transducers which are capable of transmitting and receiving pressure waves directed into a medium such as the human body. Such transducers normally comprise multielement piezoelectric materials, which vibrate in response to an applied voltage to produce the desired pressure waves. Piezoelectric transducer elements are typically constructed of lead zirconate titanate (PZT), with a plurality of elements being arranged to form a transducer assembly. A new generation ultrasonic transducer known as a micro-machined ultrasonic transducer (MUT) is also available. MUTs are typically fabricated using semiconductor manufacturing techniques with a number of elements typically formed on a common substrate to form a transducer assembly. Regardless of the type of transducer element, the transducer elements may be further assembled into a housing possibly containing control electronics, the combination of which forms an ultrasonic probe. The ultrasonic probe may include acoustic matching layers between the surface of the various types of elements and the probe body. Ultrasonic probes may then be used along with an ultrasonic transceiver to transmit and receive ultrasonic pressure waves through the various tissues of the body. The various ultrasonic responses may be further processed by an ultrasonic imaging system to display the various structures and tissues of the body.
To obtain high quality images, the ultrasonic probe must be constructed so as to produce specified frequencies of pressure waves. Generally speaking, low frequency pressure waves provide deep penetration into the medium (e.g., the body), but produce poor resolution images due to the length of the transmitted wavelengths. On the other hand, high frequency pressure waves provide high resolution, but with poor penetration. Accordingly, the selection of a transmitting frequency has involved balancing resolution and penetration concerns. Unfortunately, resolution has suffered at the expense of deeper penetration and vice versa. Traditionally, the frequency selection problem has been addressed by selecting the highest imaging frequency (i.e., best resolution) which offers adequate penetration for a given application. For example, in adult cardiac imaging, frequencies in the 2 MHz to 3 MHz range are typically selected in order to penetrate the chest wall. Lower frequencies have not been used due to the lack of sufficient image resolution. Higher frequencies are often used for radiology and vascular applications where fine resolution is required to image small lesions and arteries affected by stenotic obstructions.
Recently, new methods have been studied in an effort to obtain both high resolution and deep penetration. One such method is known as harmonic imaging. Harmonic imaging is grounded on the phenomenon that objects, such as human tissues, develop and return their own non-fundamental frequencies, i.e., harmonics of the fundamental frequency. This phenomenon and increased image processing capabilities of digital technology, make it is possible to excite an object to be imaged by transmitting at a low (and therefore deeply penetrating) fundamental frequency (fo) and receiving reflections at a higher frequency harmonic (e.g., 2fo) to form a high resolution image of an object. By way of example, a wave having a frequency less than 2 MHz can be transmitted into the human body and one or more harmonic waves having frequencies greater than 3 MHz can be received to form the image. By imaging in this manner, deep penetration can be achieved without a concomitant loss of image resolution.
Transducers have been designed for transmit frequencies in the range of 2 MHz to 3 MHz for sufficient resolution of cardiac valves, endocardial borders and other cardiac structures. Lower transmit frequencies have been used for Doppler but not for B-mode imaging. Heretofore, transmit frequency selection has been determined based on the capabilities of fundamental response imaging which required relatively high fundamental frequencies in order to obtain adequate resolution for diagnostic purposes.
However, in order to achieve the benefits of transmitting at a lower frequency for tissue penetration and receiving a harmonic frequency for improved imaging resolution, broadband transducers are required which can transmit sufficient bandwidth about the fundamental frequency and receive sufficient bandwidth about the harmonic(s). The s4 transducer available with the SONOS(trademark) 5500 an ultrasound imaging system manufactured by and commercially available from Agilent Technologies, U.S.A., has a suitable bandwidth to achieve harmonic imaging with a single transducer thus providing a significant clinical improvement. Furthermore, the combination of the s4 transducer and the SONOS(trademark) 5500 provide multiple imaging parameter choices using a single transducer, thus providing a penetration choice as well as a resolution choice.
However, several problems exist with the current harmonic imaging technology due to the fact that current transducer designs have been based on fundamental imaging and not harmonic imaging. The goal with harmonic imaging is to generate harmonic signals in the body of high enough intensity to be above the noise floor of the system. Theoretically, a harmonic signal will be more than 20 dB down from the fundamental backscatter and therefore wide dynamic range receivers are required. In the near-field, where harmonic responses have not yet formed and in the far-field where attenuation has taken over, it is not uncommon for a harmonic response to be 30-40 dB down from the fundamental backscatter. It is critical that the magnitude of the harmonic signal generated in the body be over both the noise floor of the system and the transmitted second harmonic backscatter. This is difficult to attain across the entire field of view, particularly in the near-field, where harmonics have not had the time to build and in the far-field where attenuation of the signal becomes a problem. In order to improve harmonic imaging the problem of non-uniform harmonic generation needs to be addressed.
Contrast Imaging Background
Harmonic imaging can also be particularly effective when used in conjunction with contrast agents. In contrast agent imaging, gas or fluid filled micro-sphere contrast agents known as microbubbles are typically injected into a medium, normally the bloodstream. Because of their strong nonlinear response characteristics when insonified at particular frequencies, contrast agent resonation can be easily detected by an ultrasound transducer. By using harmonic imaging after introducing contrast agents, medical personnel can significantly enhance imaging capability for diagnosing the health of blood-filled tissues and blood flow dynamics within a patient""s circulatory system. For example, contrast agent harmonic imaging is especially effective in detecting myocardial boundaries, assessing microvascular blood flow, and detecting myocardial perfusion.
In addition to today""s problems with Tissue Harmonic imaging, there are similar problems associated with the imaging of contrast agents. The power or mechanical index of the incident ultrasonic pressure wave directly affects the contrast agent acoustical response. At lower powers, microbubbles formed by encapsulating one or more gaseous contrast agents with a material forming a shell thereon resonate and emit harmonics of the transmitted frequency. The magnitude of these microbubble harmonics depends on the magnitude of the excitation signal pulse. At higher acoustical powers, microbubbles rupture and emit strong broadband signals. In order to take advantage of these strong backscattered signals for imaging purposes, it is desired to have uniform destruction within the imaging plane. In general, the higher the transmitted frequency, the greater the variation in the response from the microbubble within the imaging plane. It has been determined that lower frequencies are more efficient at bubble destruction than higher frequencies.
Today""s systems, in order to deal with the lack of uniformity in bubble detection and bubble destruction, use multi-pulse techniques to increase the signal-to-noise ratio and to increase destruction of microbubbles. However, multi-pulse techniques for detection of contrast agents require the user to be able to discriminate motion artifacts from true bubble resonance and destruction signals. For example, in high mechanical index (MI) triggered techniques, such as Harmonic Power Doppler, it is critical that triggering occurs during that portion of the cardiac cycle when the heart is relatively stationary. Discriminating between motion artifacts resulting from triggering and variations in the cardiac cycle has made diagnosis difficult for users and has slowed acceptance of contrast imaging in clinical practices.
Several patents have been granted focusing on overcoming the signal-to-noise problem with harmonic imaging. U.S. Pat. No. 5,740,128 to Hossack et al. teaches a transmit element that minimizes the energy transmitted into the body at the range of frequencies where a response generated harmonic is expected as the transmitted energy can not be distinguished from a generated harmonic signal. The techniques revealed by Hossack address the dynamic range between the transmitted or fundamental frequency and the harmonic signal response. Hossack""s techniques do not address non-uniform harmonic signal responses in near-field and far-field imaging planes.
U.S. Pat. No. 5,833,613 to Averkiou et al. teaches a multi-pulse transmission signal designed to minimize transmitted noise and to increase the harmonic signal. The technique transmits consecutive pulses with reversed polarities from one another into the body. Reflective addition of the pulses will subtract transmitted second harmonic reflections (undesired) and will cause the generated harmonic waveforms, which return to the transducer in phase, to add coherently thus increasing the signal-to-noise ratio. Like U.S. Pat. No. 5,740,128 to Hossack et al., Averkiou""s multi-pulse technique does not address non-uniform harmonic signal responses. Averkiou""s multi-pulse technique is susceptible to motion artifacts generated by each subsequent return of the multiple transmission pulses. In addition, Averkiou""s multi-pulse technique increases signal-processing overhead, which leads to a decrease in frame rate for real-time ultrasound diagnostic systems.
A second U.S. Pat. No. 5,879,303, to Averkiou et al., teaches a method for ultrasonic imaging using reflections from both the fundamental and one or more harmonic signals in the presence of depth dependent ultrasound signal attenuation. The ""303 patent to Averkiou further teaches that by removing reflections from the fundamental and using only generated harmonics to create the image, multi-path clutter from undesired structures in the near-field may be removed. While the ""303 patent discusses a need to reduce multi-path clutter in the near-field, the ""303 patent fails to address the need to quickly generate harmonic signal responses in the near-field, where the generated harmonic signals are generally 30 dB down from the fundamental. In addition, the ""303 patent fails to address the need for deeper signal penetration in order to generate harmonic signal responses with a suitable energy level in the far-field where the energy is also more than 30 dB down.
U.S. Pat. No. 5,558,092 to Unger et al. discloses methods and apparatus for performing diagnostic and therapeutic ultrasound simultaneously. Unger introduces a specialized transducer with xe2x80x9cdiagnosticxe2x80x9d elements and xe2x80x9ctherapeuticxe2x80x9d elements. The therapeutic elements are intended to rupture vesicles (microbubbles) containing drugs/genes or other therapeutic materials, while the diagnostic elements are available to monitor results of the rupture events. Unger teaches low frequency high power ultrasonic signals to enhance rupturing of the vesicles for therapeutic reasons. Unger""s transducers are complicated, difficult to manufacture, and expensive. The transducers also suffer in performance from a typical phased-array transducer because the full aperature can not be used for imaging as a significant portion of the transducer is dedicated to therapeutic insonification.
U.S. Pat. No. 5,410,516 to Uhlendorf et al. discloses contrast agent imaging along with single pulse excitation techniques such as harmonic imaging. Specifically, Uhlendorf teaches that by choosing a radio-frequency (RF) filter to selectively observe any integer harmonic (2nd, 3rd, etc.), subharmonic (e.g., 1/2 harmonic) or ultraharmonic (e.g., 3/2 harmonic) it is possible to improve the microbubble to tissue ratio. The second harmonic has proven most useful due to the large bubble response at this frequency as compared to higher order integer harmonics, subharmonics or ultraharmonics. The second harmonic also is most practical due to bandwidth limitations on the transducer (i.e.,  less than 70% bandwidth, where percent bandwidth is defined as the difference of the high corner frequency xe2x88x926 dB point from the low corner frequency xe2x88x926 dB point, divided by the center frequency. However, single pulse excitation techniques together with harmonic imaging suffer from poor microbubble-to-tissue ratio as large tissue integer-harmonic signals mask the signal generated by the contrast agent.
In response to these and other shortcomings of the prior art the present invention relates to an improved ultrasonic imaging system for harmonic imaging of an object in a medium. Briefly described, in architecture, the system can be implemented with a wideband phased-array transducer, a transmitter which generates electrical signals that may be converted by the transducer to fundamental ultrasonic pressure waves for transmission into a medium, a receiver for receiving harmonic ultrasonic responses from at least one object in the medium, and a control system electrically coupled to the transmitter and the receiver for controlling operation of the transmitter and the receiver.
The present invention can also be viewed as providing a method for uniform harmonic imaging, contrast agent detection and destruction. In this regard, the method can be broadly summarized by the following steps: introducing at least one contrast agent; insonifying tissue with an ultrasound signal at a fundamental frequency less than 1.5 MHz of a sufficient magnitude such that homogenous contrast agent destruction occurs within a single pulse; receiving echoes at one or more harmonic frequencies of the fundamental frequency, filtering out harmonic responses from tissue and processing the echoes to produce an image of the object.
The present invention can also be viewed as providing a method for uniform harmonic imaging without a contrast agent. In this regard, the method can be broadly summarized by the following steps: insonifying tissue with an ultrasound signal at a fundamental frequency less than or equal to 1.5 MHz; using transmit apodization to generate tissue generated harmonic responses in the near-field; receiving echoes at one or more harmonic frequencies of the fundamental frequency; and processing the echoes to produce an image of the object.
An ultrasonic imaging system in accordance with the present invention is designed to increase harmonic generation responses in the article under observation at the expense of resolution, by creating a more uniform transmit power field throughout the entire image. Uniformity in the transmit power field due to the lower transmit frequency provides for improved tissue harmonic response imaging. After introducing one or more contrast agents into the tissue of interest, a more uniform power field provides homogeneous contrast agent microbubble destruction. In addition, a more uniform power field provides for homogeneous microbubble detection after the tissue harmonics have been filtered from the response.
The improved ultrasonic imaging system in accordance with the present invention uses a wideband ( greater than 70%, 6 dB, 2-way bandwidth) phased-array transducer technology for uniform harmonic generation and uniform microbubble detection and destruction at transmit frequencies less than 1.5 MHz. The ultrasonic imaging system displays a tissue generated harmonic response signal, which is above the noise floor of the system throughout the imagexe2x80x94not just in the mid-field. The increased signal to noise ratio throughout the image results in an image of improved quality.
In addition to improved imaging quality for insonified tissue, ultrasonic pressure wave uniformity creates a more uniform microbubble response in the ultrasound field allowing different regions to be compared in terms of concentration of the contrast agent. In turn, regional comparisons of contrast agent concentration will allow regional comparisons of perfusion, thereby permitting improved imaging of blood flow throughout the circulatory system of a patient. By using a lower fundamental frequency pressure wave transmission (e.g.,  less than 1.5 MHz), it becomes possible to destroy contrast agents uniformly with a single transmit pulse. This is due to the more uniform power field present that results from the lower fundamental frequency and from the fact that low frequencies more efficiently destroy some contrast agents. When a rapid destruction of contrast agent occurs, a broadband response is back scattered from the destroyed microbubbles. This broadband contrast response, although somewhat detectable at higher frequencies, has an increased magnitude at lower frequencies (higher signal to noise). Furthermore, newer designs of broadband transducers (e.g.,  greater than 70%) make it possible to receive and process ultrasonic responses between the 2nd and 3rd harmonics where responses from tissue are minimal and a relatively stronger microbubble response signal is present. Improved electromechanical properties have been observed with single crystals of Pb(Zn1/3Nb2/3O3xe2x80x94PbTiO3) (PZN-PT) and Pb(Mg1/3Nb2/3)O3xe2x80x94PbTiO3 (PMN-PT). Using these materials, longitudinal coupling constants, k33, of 85% to 93% have been observed as compared with conventional PZT-type ceramics, which normally exhibit a k33 value of approximately 70% to 75%. As known in the art, the coupling constant, k33, represents the efficiency of conversion of electrical energy to mechanical energy and vice versa. This high coupling of PZN-PT and PMN-PT single crystals provides the potential for improved sensitivity and bandwidth in transducer design.
Other systems, methods, features, and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.